In an electrical communications system, it is sometimes advantageous to transmit information signals (e.g., video, audio, data) over a pair of conductors (hereinafter a “conductor pair” or a “differential pair” or simply a “pair”) rather than a single conductor. The signals transmitted on each conductor of the differential pair have equal magnitudes, but opposite phases, and the information signal is embedded as the voltage difference between the signals carried on the two conductors. This transmission technique is generally referred to as “balanced” transmission. When signals are transmitted over a conductor such as a copper wire in a communications cable, electrical noise from external sources such as lightning, computer equipment, radio stations, etc. may be picked up by the conductor, degrading the quality of the signal carried by the conductor. With balanced transmission techniques, each conductor in a differential pair often picks up approximately the same amount of noise from these external sources. Because approximately an equal amount of noise is added to the signals carried by both conductors of the differential pair, the information signal is typically not disturbed, as the information signal is extracted by taking the difference of the signals carried on the two conductors of the differential pair; thus the noise signal is cancelled out by the subtraction process.
Many communications systems include a plurality of differential pairs. For example, high speed communications systems that are used to connect computers and/or other processing devices to local area networks and/or to external networks such as the Internet typically include four differential pairs. In such systems, the conductors of the multiple differential pairs are usually bundled together within a cable, and thus necessarily extend in the same direction for some distance. Unfortunately, when multiple differential pairs are bunched closely together, another type of noise referred to as “crosstalk” may arise.
“Crosstalk” refers to signal energy from a conductor of one differential pair that is picked up by a conductor of another differential pair in the communications system. Typically, a variety of techniques are used to reduce crosstalk in communications systems such as, for example, tightly twisting the paired conductors (which are typically copper wires) in a cable, whereby different pairs are twisted at different rates that are not harmonically related, so that each conductor in the cable picks up approximately equal amounts of signal energy from the two conductors of each of the other differential pairs included in the cable. If this condition can be maintained, then the crosstalk noise may be significantly reduced, as the conductors of each differential pair carry equal magnitude, but opposite phase signals such that the crosstalk added by the two conductors of a differential pair onto the other conductors in the cable tends to cancel out. While such twisting of the conductors and/or various other known techniques may substantially reduce crosstalk in cables, most communications systems include both cables and communications connectors that interconnect the cables and/or connect the cables to computer hardware. Unfortunately, the communications connector configurations that were adopted years ago generally did not maintain the conductors of each differential pair a uniform distance from the conductors of the other differential pairs in the connector hardware. Moreover, in order to maintain backward compatibility with connector hardware that is already in place in existing homes and office buildings, the connector configurations have, for the most part, not been changed. As a result, many current connector designs generally introduce some amount of crosstalk.
FIG. 1 depicts an exemplary electrical communications system in which crosstalk is likely to occur. As shown in FIG. 1, a computer 11 is connected by a cable 12 to a modular wall jack 15 that is mounted in a wall plate 19. The cable 12 contains a plurality of (typically four) differential pairs. The cable 12 further includes a modular plug 13 at each end thereof. One of the modular plugs 13 inserts into a modular jack (not pictured in FIG. 1) that is provided in the back of the computer 11, and the second modular plug 13 inserts into an opening 16 in the front side of the modular jack 15. The blades of each of the plugs 13 mate with respective contacts 1-8 (not pictured in FIG. 1) of the jack 15 into which the plug is inserted. In this manner, information signals may be communicated from the computer 11 to the modular 15. The modular jack 15 includes a connector assembly 17 at the back end thereof that receives and holds wires from a second cable 18 that are individually pressed into slots in the connector assembly 17 to make mechanical and electrical connection. The second cable 18 may connect the computer 11 to, for example, network equipment and/or the Internet.
Pursuant to certain industry standards (e.g., the TIA/EIA-568-B.2-1 standard approved Jun. 20, 2002 by the Telecommunications Industry Association), the communication system of FIG. 1 may include a total of eight information signal carrying conductors (four differential pairs). These standards also specify that, at the plug-jack mating point, the eight contacts 1-8 of the jack 15 are aligned in a row in a generally parallel, side-by-side relationship. These standards further define the specific position of the contacts 1-8 of each of the four differential pairs. The contact positions and pair assignments according to the T568B designation are shown in FIG. 2. The other designation defined in the standards, namely T568A, is similar with the exception that the assignments of pairs 2 and 3 are swapped. As shown in FIG. 2, in the plug-jack mating region where the blades of the modular plug 13 (see FIG. 1) mate with the contacts 1-8 (herein, these contacts may also be referred to as “contact wires”) of the modular jack 15, the contacts of each differential pair are not equidistant from the contacts of the other differential pairs. By way of example, contact 3 (of pair 3) is closer to contact 2 (of pair 2) than to contact 1 (of pair 2). Consequently, when the conductors of pair 3 are excited differentially (i.e., when a differential information signal is transmitted over pair 3), a first amount of signal energy is coupled (capacitively and/or inductively) onto contact 2 from contact 3 and a second (lesser) amount of signal energy is coupled (capacitively and inductively) onto contact 1 from contact 3. As such, the signals induced from contact 3 onto contacts 2 and 1 of pair 2 do not completely cancel each other out, and what is known as a differential-to-differential crosstalk signal is induced on pair 1. This differential-to-differential crosstalk includes both near-end crosstalk (NEXT), which is the crosstalk measured at an input location corresponding to a source at the same location, and far-end crosstalk (FEXT), which is the crosstalk measured at the output location corresponding to a source at the input location. Both types of crosstalk comprise an undesirable signal that interferes with the information signal. Differential-to-differential crosstalk also is induced from contact 6 of pair 3 onto contacts 1 and 2 of pair 1, although the impact of this crosstalk tends to be significantly smaller due to the much greater distance (and hence reduced coupling) between contact 6 and contacts 1 and 2. Similar differential-to-differential crosstalk arises to varying degrees with respect to each of the other differential pairs in the modular plug 13 and the modular jack 15, with the highest levels of differential-to-differential crosstalk occurring between the contacts of pairs 1 and 3 due to the interlacing of the contacts of these pairs at the plug-jack mating point (i.e., the contacts of pair 1 are sandwiched between the contacts of pair 3).
A second type of crosstalk, referred to as differential-to-common mode crosstalk, may also be generated as a result of, among other things, the industry standard defined configurations for the conductors of the four differential pairs at the plug-jack mating point. Differential-to-common mode crosstalk arises where the conductors of a differential pair, when excited differentially, couple unequal amounts of energy on both conductors of another differential pair where the two conductors of the victim differential pair are treated as the equivalent of a single conductor. By way of example, the contacts of pair 3 (contacts 3 and 6) are not spaced an equal distance from the contacts of pair 2 (contacts 1 and 2) or pair 4 (contacts 7 and 8). Specifically, contact 3 is located immediately adjacent contacts 1 and 2 in the contact region, whereas contact 6 is located some distance from contacts 1 and 2. Similarly, contact 6 is located immediately adjacent contacts 7 and 8 in the contact region, whereas contact 3 is located some distance from contacts 7 and 8. As a result, when pair 3 is excited differentially, differential-to-common mode crosstalk is induced onto both pair 2 (from contact 3 of pair 3) and on pair 4 (from contact 6 of pair 3). This crosstalk is an undesirable signal that may, for example, negatively effect the overall channel performance of the communications system. While typically the highest level of differential-to-common mode crosstalk is the common mode crosstalk that is induced from pair 3 onto pairs 2 and 4, differential-to-common mode crosstalk may also be induced between various of the other differential pairs.